A team of engineers at the University of California San Diego has built a tiny, ultra-low power injectable biosensor that could be used for uninterrupted, long-term alcohol monitoring. The chip is adequately small to be implanted in the body just under the surface of the skin and is powered wirelessly by a wearable device, such as a patch or smartwatch.

Alcohol monitoring chip is small enough to be implanted just under the surface of the skin. (Image credit: David Baillot/UC San Diego Jacobs School of Engineering)

“The ultimate goal of this work is to develop a routine, unobtrusive alcohol and drug monitoring device for patients in substance abuse treatment programs,” said Drew Hall, an electrical engineering professor at the UC San Diego Jacobs School of Engineering who led the project. Hall is also affiliated with the Center for Wireless Communications and the Center for Wearable Sensors, both at UC San Diego. Hall’s team presented this research at the 2018 IEEE Custom Integrated Circuits Conference (CICC) in San Diego on April 10.

One of the difficulties for patients in treatment programs is the absence of convenient tools for routine monitoring. Breathalyzers, presently the most standard way to estimate blood alcohol levels, are bulky devices that necessitate patient initiation and are not that accurate, Hall pointed out. A blood test is the most accurate technique, but that would necessitate a trained technician to carry it out. Tattoo-based alcohol sensors that can be worn on the skin are a favorable new alternative, but they can be easily taken off and can be used only once.

“A tiny injectable sensor—that can be administered in a clinic without surgery—could make it easier for patients to follow a prescribed course of monitoring for extended periods of time,” Hall said.

The biosensor chip measures about one cubic millimeter in size and can be injected under the skin in interstitial fluid—the fluid that encompasses the body’s cells. It has a sensor that is coated with alcohol oxidase, an enzyme that selectively interacts with alcohol to produce a byproduct that can be electrochemically detected. The electrical signals are conveyed wirelessly to an adjacent wearable device such as a smartwatch, which also wirelessly powers the chip. Two extra sensors on the chip measure pH levels and background signals. These get negated to ensure the alcohol reading is more accurate.

The researchers engineered the chip to use up as little power as possible - 970 nanowatts total, which is around one million times less power than what a smartphone consumes when making a phone call. “We don’t want the chip to have a significant impact on the battery life of the wearable device. And since we’re implanting this, we don’t want a lot of heat being locally generated inside the body or a battery that is potentially toxic,” Hall said.

One of the ways the chip runs on such ultra-low power is by conveying data via a method known as backscattering. This occurs when a neighboring device like a smartwatch transmits radio frequency signals to the chip, and the chip transmits data by adjusting and reflecting those signals back to the smartwatch. The researchers also developed ultra-low power sensor readout circuits for the chip and reduced its measurement time to merely three seconds, resulting in a lesser amount of power consumption.

The researchers tested the chip in vitro with an arrangement that imitated an implanted environment. This included mixtures of ethanol in diluted human serum under layers of pigskin.

For forthcoming studies, the team is planning to test the chip in live animals. Hall’s team is working with CARI Therapeutics, a startup located in the Qualcomm Institute Innovation Space at UC San Diego, and Dr. Carla Marienfeld, an addiction psychiatrist at UC San Diego who is an expert in treating persons with substance abuse disorders, to enhance the chip for next generation rehab monitoring. Hall’s team is creating versions of this chip that can monitor other drugs and molecules in the body.

This is a proof-of-concept platform technology. We’ve shown that this chip can work for alcohol, but we envision creating others that can detect different substances of abuse and injecting a customized cocktail of them into a patient to provide long-term, personalized medical monitoring.